This application claims priority to Japanese Patent Application No. 2002-191860, filed Jul. 1, 2002, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
This invention generally relates to an optical clock multiplier and method thereof and more specifically relates to an optical clock multiplier to double a frequency of an input optical clock and method thereof.
2. Background of the Invention
As an optical clock multiplier, a configuration to use a Mach-Zehnder interference system is well known. A configuration of this system is a to divide an input optical clock pulse into two portions, one portion is delayed from the other for a time equivalent to one half of a frequency of the input optical clock pulse and the other portion is amplitude-adjusted, and to combine the two portions.
Another well-known configuration is to input an optical clock pulse of linear polarization at an incident angle of 45xc2x0 against a birefringent medium having a differential group delay (DGD) corresponding to one half of a frequency of the input optical clock pulse to generate two polarization components temporally different from each other and extract the same polarization component of an angle of 45xc2x0 from both polarization components.
However, in these configurations, a pulse width of the multiplied output optical pulse is equivalent to that of the input optical clock pulse. Therefore, all of the conventional systems have to provide a short pulse light source to generate an optical clock whose optical pulse width is one half of a desired optical pulse width of an output optical clock. The more the frequency rises, the more it becomes difficult to provide such a short pulse light source.
As a means to shorten an optical pulse width, a configuration to serially connect electroabsorption optical modulators (EA modulators) is well known.
Since the conventional system utilizing a Mach-Zehnder interference system uses a Mach-Zehnder interferometer, it could be rather unstable. That is, it is necessary to control polarizations of lights propagating on both arms so that both polarizations become identical after combined together. Furthermore, a relative phase difference between both arms must be accurately kept within half a wavelength or less. These restrictions also lead to manufacturing difficulties in production.
In a conventional configuration to use a birefringent medium, an input optical clock pulse must be input to the birefringent medium in such a condition that its polarization angle becomes precisely 45xc2x0 to the birefringent medium. To obtain the stable operation, polarization controllers should be disposed in front and back of the birefringent medium.
When to shorten an optical pulse width using serially connected EA modulators of multi-stage, an optical amplifier is required to compensate attenuation due to the EA modulators. In an erbium-doped optical fiber amplifier generally used as an optical amplifier, a phase fluctuates easily and thus phase fluctuation (i.e. jitter and wander) of an optical pulse is likely to occur. Accordingly, it is necessary to dispose a circuit structure for automatically adjusting a phase of a modulating signal applied to the EA modulator to compensate a phase difference between the respective stages. This makes the configuration much complicated.
Moreover, in the multi-stage connection of EA modulators, it is extremely difficult to flexibly adjust an optical pulse width. Also, wavelength dependency is quite severe.
An optical clock multiplier according to the invention comprises a phase modulator to shift a phase in pulse duration of an input optical clock pulse by xcfx80, a polarization mode dispersion device, having a predetermined time difference between first and second polarizations orthogonal to each other, to divide an output light from the phase modulator into the first and second polarization components, and a polarization device to extract one of polarization components in a third polarization inclined substantially at an angle of 45xc2x0 against the first polarization and a fourth polarization orthogonal to the third polarization out of the output light from the polarization mode dispersion device.
An optical clock multiplying method according to the invention comprises steps of shifting a phase in pulse period or pulse duration of an input optical clock pulse by xcfx80; dividing the input optical clock pulse whose phase is shifted at the phase shifting step into first and second polarization components orthogonal to each other with a polarization mode dispersion device having a predetermined time difference between the first and second polarizations; and extracting one of polarization components of a third polarization inclined substantially at an angle of 45xc2x0 against the first polarization and a fourth polarization orthogonal to the third polarization out of the first and second polarization components divided at the polarization dividing step.
An optical clock multiplier according to the invention comprises a CW light source to generate a CW light of a first wavelength, a phase modulator, to which an input optical clock pulse of a second wavelength different from the first wavelength and the output light from the CW light source enter, to give a phase difference of n between pulse duration and non-pulse duration of the input optical clock pulse to the CW light, a polarization mode dispersion device having a predetermined time difference between first and second polarizations orthogonal to each other to divide the light of the first wavelength output from the phase modulator into the first and second polarization components, and a polarization device to extract one of polarization components in a third polarization inclined substantially at an angle of 45xc2x0 against the first polarization and a fourth polarization orthogonal to the third polarization out of the light of the first wavelength output from the polarization mode dispersion device.
An optical clock multiplying method according to the invention comprises a CW light generating step in which a CW light source generates a CW light of a first wavelength, a phase modulating step in which a phase modulator receives an input optical clock pulse of a second wavelength different from the first wavelength and CW light output from the CW light source and gives a phase difference of n between pulse duration and non-pulse duration of the input light clock pulse to the CW light, a polarization dividing step in which a polarization mode dispersion device having a predetermined time difference between first and second polarizations orthogonal to each other divides the light of the first wavelength output from the phase modulator into the first and second polarization components, and a step for extracting one of polarization components in a third polarization inclined substantially at an angle of 45xc2x0 to the first polarization and a fourth polarization orthogonal to the third polarization out of the light of the first wavelength output from the polarization mode dispersion device.